Confusion over acceptable oil grade to use - Ford Focus 1.8 petrol?

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In my Ford Focus (UK) petrol owner's guide it says that 5w30 engine oil should be used at all times for optimum performance. However, further down the page below where it says engine oil top-up it says that 5w30, 10w40 or 5w40 oil can be used albeit with reduced fuel economy, performance etc. Does Ford mean that e.g. 10w40 can be used for a complete oil change (as it says "top-up" it doesn't say by what amount e.g. 1 litre or more) or only for topping up. Even more confusing, the Hayne's manual for this model says that 5w30, 10w40 or 5w40 can be used but higher viscosities e.g. 15w40 could cause damage. I have read in the car mechanics magazine that the use of higher viscositites than 5w30 can cause valve or other problems later on.

So I would like to know once and for all whether 10w40 oil would cause damage to the engine

thanks
 
by damage they mean carbon build ups due to oil contamination form burning around valve seals etc. and having a possibility of sticking piston rings. the oil's used by modern engines are tested heavily for best performance. topping up with the odd litre of heavier grade oil will not in itself cause any problems a sit wil be diluted and changed at next service anyway. best not to run the heavier oils as they are not recommended for good reasons.
it will be unlikely that running say, with entirely the heavier oil will cause an engine failure but it will over time degrade the engine and it's performance. (reducing it's useful working life)
don't worry about topping up with a heavier or lighter oil providing its not excessive eg a half litre or more every month or so. again also oil's need to get to correct working temperatures to be effective and lots of short stop start journeys degrade an oil much quicker than long motorway journeys.
 
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